The origin of very wide binary stars

TL;DR

This paper proposes that very wide binary stars in the Galaxy originate from the dissolution of star clusters, supported by N-body simulations showing their formation during this process.

Contribution

It introduces a new formation mechanism for wide binary stars during star cluster dissolution, supported by simulation results and universal eccentricity and mass ratio distributions.

Findings

Wide binaries can form from dissolving star clusters.

The wide binary fraction depends on initial cluster conditions.

Most wide binaries are likely higher-order multiple systems.

Abstract

A large population of fragile, wide (> 1000 AU) binary systems exists in the Galactic field and halo. These wide binary stars cannot be primordial because of the high stellar density in star forming regions, while formation by capture in the Galactic field is highly improbable. We propose that these binary systems were formed during the dissolution phase of star clusters (see Kouwenhoven et al. 2010, for details). Stars escaping from a dissolving star cluster can have very similar velocities, which can lead to the formation of a wide binary systems. We carry out N-body simulations to test this hypothesis. The results indicate that this mechanism explains the origin of wide binary systems in the Galaxy. The resulting wide binary fraction and semi-major axis distribution depend on the initial conditions of the dissolving star cluster, while the distributions in eccentricity and mass ratio are universal. Finally, since most stars are formed in (relatively tight) primordial binaries, we predict that a large fraction of the wide "binary stars" are in fact higher-order multiple systems.